Displacement machine having a ceramic rolling bearing

ABSTRACT

The displacement machine is intended in particular for use as a pump, and has a working area, formed in a housing, which working area is limited by two end walls and a casing, and is accessible via an inlet and an outlet for a working medium. At least one rotor, movably disposed in the housing, subdivides the working area, and a guide gearing for rotor control is connected to a drive apparatus located outside the working area. The invention has as its object to propose a displacement machine which can be manufactured less expensively and with less weight than known displacement machines of this type, and in particular can be operated without any special lubricant for the bearings. This object is attained in that the rotor, or respectively the rotors, is or are, supported at least partially inside the working area, and in that the bearing or bearings, disposed inside the working area, is, or are, accessible for the working medium, and is, or are, designed as rolling bearing made of ceramic materials, making possible operation of the displacement machine without lubricants. Contamination of the working medium from lubricant is thereby practically excluded.

The invention relates to a displacement machine, in particular for useas a pump, with a working area formed in a housing, which working areais limited by two end walls and a casing, and is accessible for aworking medium via an inlet and an outlet, and at least one rotor,subdividing the working area, movably disposed in the housing, and aguide gearing for rotor control, and a drive apparatus situated outsidethe working area.

Displacement machines of this kind are known in many different types.They can be designed and used for supply of a medium under pressure, forexample air, as well as designed and used as vacuum pumps. In many areasof application of such machines, in particular in the packaging and foodindustry, the pressurized medium must not be contaminated with oil. Itis therefore essential that the machines get by without oil in theworking area. With respect to the rotors, the problem has been resolvedwith geometries which allow a contact-free engagement of the rotors, sothat the latter do not have to be lubricated with oil. More of aproblem, however, is the bearing of the rotors, as will be explained inthe following with reference to some typical examples.

In a first type of displacement machine of the initially mentioned type,the rotors are supported by means of sliding bearings. Sliding bearingsare known today which have very good dry running features, for examplethose with coatings of silicon carbide. Although such bearings arecapable of withstanding an operation without lubricant for some timewithout damage, for example during start-up of the machine or duringinterruption of the lubricant supply until the machine is stopped, theyare not suited, however, to non-lubricated continuous operation. Thereare some cases in which the medium for lubrication of the bearingsrequired by the machine can be brought in, especially if this medium isliquid. In all other cases, nothing else can be done except to seal offthe bearings with respect to the working area if the working medium issupposed to be protected against contamination from lubricants. Thistakes place in most cases by means of dynamic seals. The most importantproblems of constructions of this incomplete tightness as well aspossibly arising friction, and the heat connected therewith, as well asthe limited life of the seals. Belonging to this type of displacementmachine, for example, is the screw-type compressor described in thedocument DE 31 24 247 C1. To simplify the manufacture of this screw-typecompressor, both slides are made of ceramic material at least in theregion of the screw profile. It is also indicated in this document thatit is useful to support the slides by means of sliding bearings, thesliding bearing bush being made expediently of silicon carbide and theshaft in the bearing regions being preferably coated with ceramicmaterial. Furthermore it is expressly pointed out that a lubricant isneeded for these bearings, and that this lubricant is preferably water.

A second type of displacement machine works with hydrodynamic orhydrostatic bearings. Of course sealing problems present themselveshere, too, as soon as the working area is not allowed to be contaminatedwith lubricants. Overcoming these sealing problems requires greatercomplexity of construction, which increases the weight of the machine inan undesirable way. An example of a machine with hydrostatic bearings isdescribed in the European Patent Application EP 0 376 373 A1. In thistype of displacement machine the weight and the complexity ofconstruction are increased even further through the installationsnecessary for providing the lubricant pressure.

A third type of displacement machine is equipped with magnetic bearingsfor the rotor shaft. Such bearings have in themselves a relatively greatweight. Because of the relatively minimal forces which can be absorbedby magnetic bearings, the rotors must be driven by separate,electronically synchronized motors, and cannot be synchronized with aguide gearing. A mechanical emergency synchronization mechanism isfrequently provided, however, for the event of a failure of thesynchronizing mechanism.

Still another type of displacement machine has shafts with one-sidedbearing support, the bearing being provided on the pressure side of theworking area. German published patent application DE 195 22 551 A1 showssuch a machine. It is apparent that also with this kind of displacementmachine the complexity of construction is considerable.

A fourth, widespread type of displacement machine operates with rollingbearings which are conventionally lubricated and are dynamically sealedwith respect to the working area. In a first subtype of this kind, therotors have bearing support on both sides, as is shown, for example, inthe German patent DE 37 06 588 C1. It can clearly be seen on thedrawings of this document that with a given length of the working area,the support length between the bearings is enlarged by the seals neededfor their sealing. It is clear that, with increasing support width, thetendency for transverse vibrations of the rotors and thus the risk ofrotor contact increases. To counteract this risk, the root diameter ofthe rotors also are designed correspondingly larger. The constructiondimensions and the weight of the machine is thereby increased. Inanother machine of this kind according to German published patentapplication DE 195 13 380 A1, the bearing is achieved on one side insidethe rotor, which has a bore instead of a bearing journal for thispurpose. The support width between the bearings is thereby decreased,but the expenditure for sealing the bearings with respect to the workingarea is not reduced. A second subtype of the aforementioned fourth typeworks with rotors with one-sided bearing support. An application for aSwiss patent for such a machine was filed by the applicant on Jul. 15,1997, under the number 1737/97. This machine has the particularadvantage that only a single bearing must be sealed, and to be moreprecise, the bearing on the pressure side of the working area. Thissealing of the working area presents fewer problems also because therisk of a contamination of the working medium is much smaller with thepressure-side seal than with the suction-side. Compared to theaforementioned, however, no construction volume is saved through thisconstruction, whereby application is limited to smaller pumps.

The present invention has as its object to propose a displacementmachine of the initially mentioned type which can be manufactured lessexpensively and with less weight than known displacement machines ofthis type, and in particular can be operated without special lubricantsfor the bearings. This object is attained in that the rotor or rotors isor are supported at least partially inside the working area, and in thatthe bearing or bearings disposed inside the working area is or areaccessible for the working medium and is or are designed as rollingbearings made of ceramic materials, whereby a lubricant-free operationof the displacement machine is possible. The machine according to theinvention can thus be operated without special lubricants for thebearings, and a contamination of the working medium with lubricant isthereby practically excluded.

In one embodiment of the invention, two rotors (8, 9), having parallelaxes and meshing with one another in external engagement, are rotatablydisposed in the housing (1), and each rotor has a shaft end, withdynamic sealing, led out on one side through a bore in the first endwall (3), and is supported on the opposite end face by a ceramic rollingbearing (16) which is fixed on the inside to the second end wall (4),designed as a blind cap, and is protected from the working area througha friction-free labyrinth seal (18) or is freely accessible. In thisembodiment, the need for sealing on both sides is eliminated, makingpossible a reduction of the support widths between the bearings. Theexpenditure during construction of the machine is thereby reduced aswell as the weight of the machine and its construction volume.

In an especially preferred embodiment of the inventive displacementmachine, for use as a vacuum pump, having the features cited in thepreceding paragraph, the geometry of the rotors is screw-shaped orscrew-like, and the machine thus operates with a primarily axialdirection of conveyance, and the inlet (6) is provided on the blind capend wall (4) in the vicinity of the ceramic rolling bearing (12) and theoutlet (7) on the shaft-end side. This preferred embodiment allows asimple, two-sided bearing support of the rotors with a reduction of thesupport widths as well as an operation without suction-side dynamicseals and a general construction volume reduction. Doing withoutsuction-side seals is particularly advantageous with vacuum pumpsbecause a failure of such a seal would lead to a contamination of theworking medium or to collapse of the vacuum in a machine withconventionally lubricated bearings.

In another embodiment, having the features cited in either one of thepreceding paragraphs but where the rotor bearings (17) are likewiseformed by inner-situated ceramic rolling bearings on the end wall (3)with the shaft lead-throughs, a lubricant-free operation of the machineis likewise made possible as well as a further reduction of the rotorsupport widths. Above and beyond this, the dynamic shafts are accessiblefrom outside and are replaceable in this embodiment.

The state of the art and a special embodiment of the invention will beexplained more closely in the following, with reference to the attacheddrawings:

FIG. 1 is a longitudinal section through a displacement machineaccording to the state of the art, and

FIG. 2 is a longitudinal section through an embodiment example of thedisplacement machine according to the invention.

FIG. 1 shows a longitudinal section through a displacement machineaccording to the state of the art, which is intended for use as a pump.Formed in a housing 1 is a working area 2, which is limited by two endwalls 3, 4 and a casing 5. Via an inlet 6, the working medium, forexample air, is sucked into the working area and is expelled therefromvia an outlet 7. Rotatably disposed in the working area are two rotors8, 9, provided in a known way with screw-shaped profiles, engaging inone another, on their generated surface. A guide gearing 10, disposedoutside the working area and driven by a drive apparatus 11, ensuresthat the two rotors rotate in opposite rotational directions withouttouching. The rotors 8 and 9 are supported with two conventional rollingbearings 12 and 13 each in the end walls 3, or respectively 4, and aresealed off with respect to the working area 2 with seals 14 and 15. Theresultant support width with this configuration is designated by L1 inFIG. 1.

In the displacement machine according to the invention, of which oneembodiment example is depicted in FIG. 2, the rotors 8 and 9 aresupported by means of rolling bearings 16 and 17 in the working area 2.Since these rolling bearings are constructed with ceramic materialswhich can achieve a high life without lubrication, or respectively withlubrication by means of the working medium alone, the seals 14 and 15shown in FIG. 1 can be done away with. The rotors thus having bearingsupport in the working area, the considerably smaller support width L2,compared to the state of the art shown in FIG. 1, being the result. Theworking area 2 needs only to be sealed off on the side of the guidegearing 10, which takes place in the present example through the dynamicfriction-free labyrinth seal 18 disposed on the side of the bearing 17remote from the working area.

What is claimed is:
 1. A displacement machine, in particular for use as a pump, comprising: a housing with a working area formed therein limited by two end walls and a casing, an inlet and an outlet via which access is provided for a working medium, at least one rotor, subdividing the working area, movably disposed in the housing, at least one bearing, a guide gearing for rotor control, and a drive apparatus situated outside the working area, wherein the at least one rotor is disposed at least partially within the working area, and the at least one bearing, supported within the working area, is accessible to the working medium and is designed as a rolling bearing made of ceramic materials, making possible a lubricant-free operation of the displacement machine.
 2. The displacement machine according to claim 1, wherein two rotors, having parallel axes and meshing with one another in external engagement, are rotatably disposed in the housing, and each rotor has a shaft end, with dynamic sealing, led out on one side through a bore in the first end wall, and is supported on the opposite end face by a ceramic rolling bearing which is fixed on the inside to the second end wall, designed as blind cap, and is protected from the working area through a friction-free labyrinth seal or is freely accessible.
 3. The displacement machine according to claim 2, wherein the rotor bearings are likewise formed by inner-situated ceramic rolling bearings on the end wall with the shaft lead-throughs.
 4. The displacement machine according to claim 2, for use as a vacuum pump, wherein the geometry of the rotors is substantially screw-shaped, and the machine thus operates with a primarily axial direction of conveyance, and the inlet is provided close to the blind cap end wall in the vicinity of the ceramic rolling bearing and the outlet on the shaft-end side.
 5. The displacement machine according to claim 4, wherein the rotor bearings are likewise formed by inner-situated ceramic rolling bearings on the end wall with the shaft lead-throughs. 